1. Field of the Invention
This invention generally relates to the high speed transmission of digital data over unconditioned voice grade telephone lines connecting distant data terminals. More specifically, this invention concerns data modems for enabling two independent data systems to be concurrently operated and serviced by a single pair of modems.
2. Description of the Prior Art
The high speed transmission of digital data between physically distant data terminals is today commonplace. Modern modems or data sets used for digital data transmission now permit such data to be transmitted over unconditioned voice grade telephone lines.
Typically, a modem is required to be used at each data terminal to modulate data to be transmitted from a transmitting terminal and demodulate received data at a receiving terminal. Each modem hence includes two basic portions, a transmitter and a receiver. The transmitter generally operates to accept digital signals and convert these digital signals to modulated analog signals for transmission over the telephone lines. The receiver operates to accept the modulated analog signals and reconvert, or otherwise recover, the originally transmitted digital signals. Both the transmitter and the receiver may be encased within a common container to share a power supply and other general purpose components such as a clock source, etc. However, since the transmitter and receiver are functionally independent and would be both functioning only during full-duplex operation, each may be separately contained.
An assortment of modulation techniques have been used by different modem manufacturers to successfully accomplish the desired data transmission. Differential phase modulation is one such technique that has been used to compensate for difficulties attendant to, and thereby permit, the use of unconditioned voice grade telephone lines. An extensive and detailed description of such differential phase modulation as applied to modems is included in U.S. Pat. No. 3,524,023, U.S. Pat. No. 3,590,381 and U.S. Pat. No. 3,643,023.
As is well known, digital data transmission may occur at any of several bit rates. Conventional modems are available to accommodate data rates ranging from below 300 bits per second to as high as 9600 bits per second. Some modems are adapted to operate at several different data rates, any one of which may be selected for a particular operation. Other modems are designed for operation at a single data rate that is compatible with other components of a data system. For example, where data sources capable of data rates of 2400 bits per second are used, a modem designed for that rate may be used. Updating of a data system to operate at a higher speed such as at 4800 bits per second may typically require a change or modification of the data sources as well as the modems. Needless to point out, the cost of a high speed modem is significantly higher than lower speed modems. For example, a modem that operates at a data rate of 4800 bits per second may have a cost that is greater than three times the cost of a modem that operates at 2400 bits per second.
As may now be readily appreciated, two complete data systems would require the use of four modems, a pair of each data system. Similarly, two complete sets of telephone lines would be required. Obviously, if two such independent data systems are able to concurrently employ a single pair of modems, a vast savings of both cost and equipment requirements would result, since certain requirements would be halved, i.e., only one pair of modems and one set of telephone lines would be required.
Accordingly, it is the intention of the present invention to provide a multiplexed data modem which will concurrently accommodate two independent data systems and thereby substantially reduce the normally associated equipment requirements and attendant costs.